Diabetes is a leading cause of morbidity and death in the United States. Obesity is a major risk factor for the most common form of diabetes, type 2 diabetes, which is characterized by resistance to the actions of insulin. We have discovered a novel, secreted protein called resistin that is adipocyte-specific and circulates at elevated levels in obesity. Hyperresistinemia impairs glucose tolerance, and lack of resistin improves hyperglycemia and insulin resistance in mice with diet induced obesity. We hypothesize that 1) resistin will alter insulin action and cardiovascular disease in genetic models of obesity and atherosclerosis;2) cellular actions of resistin involve induction of SOCS-3 and/or inhibition of AMPK, mediated by discrete biochemical forms of resistin;and 3) that the effects of mouse resistin are translatable to the human. These hypotheses will be directly tested in the experiments proposed in this project. Specific Aim 1 is to determine the effects of resistin deficiency in genetic models of obesity and atherosclerosis. We hypothesize that mice lacking resistin will be protected from obesity-associated diabetes and atherosclerosis, and will test this by crossing resistin knockout mice with leptin-deficient ob/ob mice and LDL-receptor null mice, respectively. Specific Aim 2 is to understand the molecular and cellular determinants of resistin signaling. We will systematically test the importance of resistin dimerization in a variety of cell types, focusing on potential mechanisms by which resistin influences glucose metabolism that were cellular assays in different cell types, focusing on the inhibition of AMPK, as well as the activation of SOCS-3 in several cell types. Specific Aim 3 is to derive and characterize humanized mouse models of resistin expression and physiology. One of the major questions about resistin concerns the translation of the insights from mouse models to humans. Mouse resistin is derived exclusively from adipose tissue, whereas macrophages are a major source of resistin in humans. Preliminary data suggest that human and mouse resistin signal similarly in mouse cells. Human resistin will be expressed in transgenic mice from a liver specific-transgene as well as the human promoter which, in humans, expresses resistin primarily in macrophages. These studies will test the hypothesis that human resistin functions in the mouse, and will provide novel in vivo systems to determine whether human resistin is a potential mediator of insulin resistance. Together, the proposed studies will address critical questions about the role of resistin as a link between obesity, insulin resistance, and diabetes, and a potential target for intervention in these devastating diseases. These studies have important implications for our society in which diabetes and obesity are rampant.